The roles of morphogenetic differentiation in controlling the phenotypic expression of neoplastic transformation, the degree of malignancy of tumor, and the susceptibility of developing organs to carcinogenesis are studied using organ culture and tissue transplantation techniques, with current emphasis on the kidney. A defined medium for growth of rat and mouse ureteric bud epithelium in monolayer culture has been developed in which epidermal growth factor and selenium have proved essential and insulin, hydrocortisone, and transferrin have proved highly beneficial. Cell lines were successfully established from fetal rat renal mesenchyme in serum-containing media, but such lines showed karyotypic abnormalities and could not be induced to form tubular epithelium. Serum-free media containing endothelial cell growth supplement appears to offer a solution to this problem. The ability of transplacentally administered carcinogens to induce genotoxic damage in cells of embryos or fetuses exposed at different stages of gestation was determined for rat, mouse, and Syrian hamster. Cells were isolated from exposed embryos and gene mutations at two to three loci (resistance to ouabain and 6-thioguanine and to diphtheria toxin in the hamster) were assayed in vitro with simultaneous determination of survival ability. Organ specificity of induced gene mutation is being determined in embryonal cells isolated from organs of various species exposed in utero at comparable stages of gestation. A maximum level of mutation induction was found to be induced by N-nitrosoethylurea at day 9 of gestation from mesenchymal cells of the Syrian hamster fetus, with a further, much higher sensitivity very early in gestation in the immediate post-implantation period. Cells derived from the brain of fetuses treated at different times of gestation also demonstrate a similar sensitivity. Evidence indicates that cells derived from other tissues have maximum levels of mutation induction at 6 to 7 days of gestation.